Is there a way to make comments in the node editor?
Since version 2.74, blender has has the ability to display text blocks in the node editor.See the manual.

Add a frame node, then in the properties region add the text block you want to display. (A text block is simply a file made in blender's text editor.)

The frame will display the text exactly as it is in the text editor without word wrapping, you have to add the line brakes in manually (by pressing Enter at the end of the line).
This is the easiest way to make notations in the node editor.